1. Field of the Invention
The present invention relates to a resin molding die for use in the molding-based resin encapsulation of a semiconductor device, and a production method for semiconductor devices employing such a resin molding die. In particular, the present invention relates to a production method for semiconductor devices employing a high-fluidity liquid thermosetting resin as an encapsulation resin, and a resin molding die for use with such a production method.
2. Description of the Related Art
Conventionally, resin encapsulation of semiconductor devices is achieved by: (1) casting molding, (2) transfer molding, or (3) injection molding.
(1) Casting molding. This technique involves: pouring a liquid thermosetting resin into a curing cast of PPS (polyphenylene sulfide), TPX (transparent plastic material for optics), or the like; mounting semiconductor devices and inserting wire-bonded lead frames; and heating the complex in an oven.
(2) Transfer molding. Under this technique, a resin called “B stage resin”, which results after the reaction of a thermosetting resin is interrupted halfway, is used. At molding, this resin is liquefied by the application of heat and pressure, poured into a heated casting die, and then allowed to thermally cure.
(3) Injection molding. This technique chiefly employs a thermoplastic resin as an encapsulation material. The encapsulation material is liquefied under a high pressure, poured into a heated casting die, and then allowed to cool.
Conventionally, the injection molding technique has only employed thermoplastic resins. However, the development of liquid thermosetting resins which have an faster reaction rate has allowed an injection molding technique employing a liquid thermosetting resin.
However, the aforementioned conventional techniques have the following problems.
(1) When the casting molding technique is used to mold an epoxy resin (a chief material which is currently used as an encapsulation resin), there is a problem in that the epoxy resin requires a long curing time for a poly addition reaction to occur; this step bottlenecks the production efficiency. Recently, radical reaction type resins which require a relatively short curing time are also studied. However, since this type of resins exhibit a substantial sink due to cure-related shrinkage, such resins have not been put to practical use.
(2) Since the transfer molding technique uses a B stage resin, i.e., a resin which results after the reaction of a thermosetting resin is interrupted halfway, such a resin must be stored in a frozen state, adding to the transportation and storage costs. Such a resin also requires a long curing time for a polyaddition reaction to occur.
(3) When the injection molding technique is used for encapsulating a semiconductor device with a thermoplastic resin, the high resin injection pressure used may break the wires which are used for wire-bonding a semiconductor device and a lead frame. In order to prevent this problem, a method has been proposed (Japanese Publication for Opposition No. 4-40870) in which a low-pressure injection is first performed to produce resin for protecting the wires, and then the actual molding of an external shape follows. However, the proposed technique requires some consideration in connection with a molding cycle time, and an increased number of molding dies must be used; therefore, this technique has yet to be introduced to actual mass production lines. Moreover, the high level of heat resistance and humidity resistance which is required for a semiconductor device encapsulation resin cannot be expected from a transparent thermoplastic resin due to its resin characteristics.
On the other hand, when the injection molding technique is used for encapsulating a semiconductor device with a liquid thermosetting resin, it is possible to protect wires by previously allowing a resin to adhere to the wires and curing the resin (called a “pre-dip curing technique”). However, according to this technique, the viscosity of the resin has been lowered due to the die heat when the resin is injected at a high injection pressure. As a result, the resin may leak from the parting faces of the molding die, allowing flashes to form.